Inulin is a type of polysaccharides, widely distributed in the natural world, and is known to be present in a colloidal form in the tubers of Asteraceae plants such as dahlias, Jerusalem artichokes, and wild chrysanthemums, and chicory roots. The characteristics of inulin are different from those of starch, such as the fact that inulin is dissolved in warm water and has a structure wherein D-fructofuranose is sequentially polymerized by dehydration onto the fructose side of sucrose via β-(2→1) linkages. The polymerization degree differs depending on the chain length of fructoses. In the case of inulin derived from a plant, the polymerization degree is in a range between about 8 to 60, and the average polymerization degree (average degree of polymerization) is described as being between 32 and 34 according to The Iwanami Dictionary of Biological Science (Iwanami Seibutsugaku Jiten, IWANAMI SHOTEN, 2nd edition (1978)) and being approximately 30 according to the Iwanami Dictionary of Physics and Chemistry (Iwanami Rikagaku Jiten, IWANAMI SHOTEN, 3rd edition (1979)).
Inulin has attracted attention as dietary fiber because it is water-soluble and is difficult to digest. Inulin further possesses an effect of promoting the growth of Lactobacillus bifidus, and thus its demand is growing in combination with the health-oriented boom in recent years. Inulin has been mainly produced abroad conventionally. Abroad, it is produced by cultivating plants such as chicory and Jerusalem artichoke and drying the juice extracted from the root stocks, and is utilized as a general food material. On the other hand, in Japan, commercial cultivation of these plants is difficult, so that inulin is not produced.
Hence, acquisition of inulin must depend on imports, and the price is more expensive than other domestic substances having a similar function, which is a barrier to industrial applications. Moreover, there are other problems in using inulin derived from a plant: the yield of inulin varies dependent on harvest conditions because its raw material is a plant; and, in the case where extraction is not performed immediately after harvesting, inulin content decreases due to self-digestion and the like.
Furthermore, in the case of inulin derived from a plant, since it is commercialized by roughly fractionating juice extracted from the plant and then spray-drying the product, the polymerization degree of inulin varies depending on the characteristics of the original plant. Therefore, the polymerization degree in the fructose chain ranges widely, and inulin having dispersed polymerization degrees (polymerization degree range: approximately 8 to 60) is obtained, resulting in lack of uniformity. For example, Critical Reviews in Food Science and Nutrition, 35(6), 525-552 (1995) discloses results showing the polymerization degrees of inulin from various plants (dahlia, chicory, and Jerusalem artichoke) obtained by HPAEC-PAD chromatography. A number of peaks were confirmed over a wide range in the polymerization degree between approximately 10 and 60 for inulins derived from plants, clearly suggesting their lack of uniformity. This problem would be solved by generating inulin showing a distribution of polymerization degree with high rate in a specific polymerization degree according to its use. However, this is very difficult.
In utilizing inulin, when an inulin fraction with an extremely high polymerization degree is used, its solubility in water is poor, resulting in an unfavorable situation upon actual application.
As a method of producing inulin in addition to the above extraction method from a plant, there exists a method of chemically producing inulin or inulin analogs utilizing inulin synthase. For example, M. Luscher et al (FEBS letter 385, 39 (1996)) reported a method of generating inulin from sucrose utilizing an enzyme obtained by extraction from a plant. This method use the coordinated action of 2 types of enzymes, sucrose: sucrose 1-fructosyltransferase (SST) and β-(2→1) fructan: β(2→1) fructan 1-fructosyltransferase (FFT). However, the preparation of these enzymes in large quantities from plant bodies requires time and effort, so that the use of this method on an industrial scale is not realistic.
Further, a method of producing an inulin analog, which causes the enzyme of a microbe to act, has been reported. For example, a method of obtaining a substance having an inulin-type structure, which involves treating conidiospores or cells of Aspergillus sydowi was disclosed (J. Biol. Chem., 43, 171(1920); Agric. Biol. Chem., 37, (9), 2111, (1973); JP Patent Publication (Kokai) No. 61-187797 A (1986); JP Patent Publication (Kokai) No. 5-308885 A (1993)). Furthermore, it was reported that an enzyme produced by microbes belonging to the genus Aspergillus or Fusarium, and another enzyme produced by microbes belonging to Streptococcus mutans generates an inulin analog, respectively (JP Patent Application No. 55-40193; Acta. Chem. Scand., B28, 589). The substances generated by the use of the enzymes produced by these microbes have structures analogous to that of inulin. However, compared with inulin derived from a plant, their molecules are quite large, or they differ in their binding forms, thus the above methods are not methods of generating inulin.
A method of generating inulin utilizing an enzyme derived from a microbe was disclosed in our co-pending patent application, PCT/JP01/01133. This is a method of producing inulin with relatively uniform polymerization degrees, which involves using sucrose as a raw material and causing a novel inulin synthase to act on it. With this method, the purpose of generating inulin utilizing an enzyme from a microbe has been achieved, and inulin with relatively uniform average polymerization degrees (average polymerization degree: 8 to 20) compared with inulin extracted from a plant could have been obtained. However, a means for effectively obtaining only inulin with a predetermined polymerization degree has not been established.